The invention relates to power supplies and battery chargers for portable defibrillators.
Portable defibrillators, such as the PD 1400 Series Products available from Zoll Medical Corporation of Burlington, Mass., deliver high energy shocks to a patient's chest for defibrillation. Typically, portable defibrillators employ batteries (e.g., lead acid batteries) to store power for generating such shocks. Eventually, the power stored in the batteries is used, and the batteries must be recharged from a source of AC power.
AC power supplies, which convert AC power to DC power, have been used for operating a portable defibrillator and for recharging the batteries in the portable defibrillator. Approaches to using AC power supplies include: building an AC power supply into the portable defibrillator; supplying an AC power supply in a separate box that includes a cord for attachment to the portable defibrillator; attaching the portable defibrillator into a "docking station" that contains an AC power supply (so that the combination of the docking station and the portable defibrillator is not itself portable); and removing the batteries from the portable defibrillator for charging in a separate AC powered unit (in which case the AC powered unit does not power the defibrillator). The conventional AC power supply is a "controlled voltage" supply that maintains a controlled, generally constant, DC output voltage for varying electrical load.
Controlled current supplies, which maintain a controlled, generally constant, DC output current for varying electrical load, have been used in other types of devices in which batteries need to be charged. Controlled current supplies offer advantages over controlled voltage supplies in that controlled current supplies can more rapidly charge batteries and can provide a more reliable indication that a battery is fully charged. Controlled current supplies are particularly well suited to charging lead acid and other types of batteries that tend to lose the ability to retain a charge when put into use before being fully charged. When such batteries are charged with a controlled voltage supply, the user sometimes will think that the battery is fully charged, both because charging takes so long (e.g., sixteen hours) and because the charger cannot reliably inform the user whether charging is complete, with the result that partially charged batteries are inadvertently put into use. Over time, use of partially charged batteries results in the batteries losing their ability to retain a charge. Though providing superior charging characteristics with regard to the time required to charge a battery, previous controlled current chargers have required complex support circuitry to implement them as AC power supplies for defibrillators. The difficulty with implementing controlled current chargers is that they must maintain a measure of the current supplied to the battery, and therefore, without support circuitry, require a known charging current into the battery to perform properly. When a defibrillator is operating, however, current levels may vary from as low as 500 milliamps up to eight amps. Thus, for the controlled current charger to be effective, support circuitry was needed to sense and integrate the current supplied to the defibrillator (as opposed to the current supplied to the battery) by the charger so that the charger could maintain a measure of the current supplied to the battery.